1. Field of the Invention
The present invention concerns a method to correct distortions in diffusion-weighted MR images, and a non-transitory, computer-readable data storage medium designed to implement such a method.
2. Description of the Prior Art
In diffusion-weighted EPI imaging (“echoplanar imaging”), distortions that are generated by unwanted eddy currents in gradient coils present a large challenge or problem. The high gradient amplitudes used to switch (activate) diffusion gradients encounter a high sensitivity (approximately 10 Hz per pixel in the phase coding direction is typical in EPI) with regard to static and dynamic field disruptions.
In diffusion imaging, multiple MR images with respective different diffusion directions and diffusion weightings, which are characterized by what is known as the b-value, are normally acquired and combined with one another in order to create parameter maps from these, for example. The parameter maps contain information about the ADC (“Apparent Diffusion Coefficient”) and the FA (“Fractional Anisotropy”).
Diffusion imaging and the information derived therefrom can be used for diagnostic purposes. However, it must be taken into account that the eddy current fields generated by the diffusion gradients lead to distortions in the MR images depicting the diffusion. The appearance of these image distortions depends on both the amplitude of the gradients (i.e. on the diffusion weighting or the b-value) and on their direction (i.e. the diffusion direction, thus the direction in which the diffusion is detected). If the acquired individual MR images are combined with one another without correction, the various distortions for the respective MR image lead to incorrect associations of pixel information, and therefore to corresponding errors, or at least to a reduced precision of the derived or calculated parameters.
According to the prior art (see for example “Correction for Distortion of Echo-Planar Images Used to Calculate the Apparent Diffusion Coefficient”, J. C. Haselgrove et al., MRM 36: 960-964 (1996)), a reference image that is suitable as a reference due to negligible eddy current-dependent distortions is generated with a relatively small b-value (for example b=0 to 200 s/mm2). However, the contrast of the reference image deviates relatively strongly from the contrast of the diffusion-weighted MR images to be corrected. For example, the signal of the cerebrospinal fluid (liquor) is brightly visible in the reference images but is nearly at the level of the background noise in the distorted diffusion-weighted MR images. An affine registration of such images therefore frequently leads to an incorrect scaling in that the distorted MR images are enlarged such that the outer contour of these MR images (which is defined by the grey brain matter) is mapped to the outer contour of the reference images (which is defined by the cerebrospinal fluid).
Moreover, according to the prior art (see for example “Eddy Current Correction in Diffusion-Weighted Imaging Using Pairs of Images Acquired With Opposite Diffusion Gradient Polarity”, N. Bodhammer et al., Magnetic Resonance in Medicine 51: 188-193 (2004)) it is known to register two inversely distorted images. This method avoids the weakness of the method according to the prior art that is described above in that two images with identical contrast but inverted distortion (due to the inversion of the gradient direction) are registered with one another. However, for this purpose at least two images must be acquired for each diffusion direction and diffusion weighting. Moreover, according to this method two relatively strongly diffusion-weighted MR images with a low signal-to-noise ratio are registered to one another, which disadvantageously affects the stability of the method.